In the field of a-c motor control it is now common practice to utilize an inverter for energizing the motor, and any of various d-c voltage sources to supply the inverter. By regulating the level of the d-c voltage supplied to the inverter the amplitude of the a-c voltage passed to the motor is regulated. By controlling the firing times of the semiconductor switches in the inverter the frequency of the a-c voltage passed to the motor can be controlled. For certain conditions, such as the maintenance of a constant torque output, it is common practice to operate the system so as to maintain substantially constant the ratio of the voltage amplitude to the frequency of the same a-c voltage supplied to the motor; generally this is termed "constant volts-per-cycle" operation. When this system is utilized, during start-up or other conditions where the d-c voltage supplied to the inverter is low, those inverters which utilize capacitors for commutation may be charged to a level insufficient for effective commutation. Accordingly the capability of the inverter to commutate a given value of load current decreases as the level of the d-c bus voltage is decreased.
Those skilled in the art have appreciated that to remedy this situation an external charging circuit can be provided; sometimes this is termed an auxiliary or precharge circuit. Such arrangements have been implemented for the simpler inverter configurations. However, for a more complex arrangement in which a pair of series-coupled main SCR's are utilized to switch the load currents, and a pair of auxiliary SCR's are connected in a circuit to commutate the main power handling SCR's, a practical external charging arrangement has not yet been developed. It is therefore a primary consideration of this invention to provide an effective external charging circuit for an inverter circuit utilizing auxiliary SCR's in the commutation of the main SCR's.